Stir together all mayonnaise ingredients in a small bowl. Lay out rye bread slices and spread mixture on each slice. Layer half of slices with corned beef, cheese, red onion, cucumber and tomato. Top with remaining bread slices. Brush both sides of sandwiches with olive oil. Heat a nonstick griddle or skillet over medium high heat. Place sandwich in hot pan and cover with foil. Place a heavy cast iron skillet on top of sandwich and cook about 3 minutes. Flip sandwich, cover with foil again, and top with skillet. Cook another 3 minutes, or until cheese melts and sandwich is crispy. Instead of this method, use a panini grill pan. Remove sandwiches to plates then cut in half and serve hot. Good served with big dill pickles.